Archaeal polar lipids are novel to the domain of life Archaea and are characterized as isoprenoid ether lipids of opposite sn-2,3 stereochemistry (12). Archaeosome vaccine adjuvants comprising natural lipid mixtures extracted from an archaeon have previously been disclosed. Such adjuvants alleviate the need for effective induction of humoral (Th2), cell-mediated (Th1), and particularly CD8+ cytotoxic T cell responses (CTL) to an antigen (25, 28). The disclosed lipids are restricted to the polar lipids extracted from archaeal biomass.
Best long-term adjuvant activity and memory responses occurred with archaeosomes prepared from the total polar lipids (TPL) of Methatzobrevibacter snzithii and Thermoplasma acidophilum (16), both of which have challenges in their preparation. M. smithii is an obligate anaerobe, requiring specialized medium for growth, including toxic sulfides and potentially flammable, explosive levels (80%) of hydrogen gas. Methanogens such as M. smithii must be protected from the lethal effects of oxygen in air. Thermoplasma and Sulfolobus TPLs consist of about 90% caldarchaeol membrane-spanning lipids that hydrate to form archaeosomes at low recoveries, in our experience of about 10-20%. Purification of lipids from lipid extracts is uncertain, tedious and costly. A more efficient, cost-effective method to produce archaeal polar lipids would be a great advantage.
Further, use of archaeal TPL natural mixtures limits the adjuvant composition to only those lipids, and in the proportions, that are extracted from archaeal species. This approach is likely to achieve stable archaeosomes but may not be optimal for a selected application. The lipid composition of the vaccine may theoretically determine whether protection occurs or not, based on the type of immune response (MHC class I or II presentation—FIG. 1A, or systemic versus mucosal) that is generated to the antigen.
For human applications the lipid mixture used for archaeosome formation needs to be defined and reproducibly produced from each batch of biomass grown and extracted. Recovery of mixtures of often 10 or more different polar lipids in each TPL extract is typical. Production benefits, including more control of head group in/out orientation on the archaeosome surface, as well as ease of obtaining regulatory approval for human use of defined and simplified compositions, may be anticipated.
Natural archaeal core lipids are predominantly of two types; namely, archaeol and its dimer called caldarchaeol (FIG. 1B). Both of these core lipids or their analogues may be synthesized chemically (3, 21). However, chemical synthesis of archaeal core lipids is complex and must consider the problems of producing mixed stereoisomers (methyl groups of archaeal isoprenoid chains are R) and of generating unwanted chemical by-products. In addition, the archaeal sn-2,3 stereochemistry must be adhered to.